Target positioner for accelerator external beam



4, 1969 NE ET AL 3,431,502

TARGET POSITIONER FOR ACCELERATOR EXTERNAL BEAM Filed 'April 10, 1968 Sheet 1 of 2 INVENTORS KENNETH F. STONE LESLIE T. JACKSON ATTORNEY March 4, 1969 sf E ET AL 3,431,502

TARGET POSITIONER FUR ACCLLIJRAIOR LXTBRNAI. BEAM Filed April 10. 1968 Sheet 2 of POWER SUPPLY w can s;

r\ smog L f sm 3 0192 awg 0 E t 'Z o \N 0% Em k E O ia: 0: r3 P g XNVENTORS \JES g KENNETH F. STONE 2E 31411 By LESLIE T. JACKSON 5 mcLE ATTORNEY United States Patent Ofice 5 Claims ABSTRACT OF THE DISCLOSURE A target positioning apparatus of the flip-coil type for use in extracted beams from a charged particle accelerator, having a high speed coil drive which is independent of the accelerator magnetic field. A unique crank arm linkage from a drive motor to the target mount provides automatic damping of the target motion at the rest positions thereby preventing damage to a fragile target.

Background of the invention This invention relates generally to charged particle accelerators and more particularly to apparatus for placing target materials into and out of a beam of charged particles which has been extracted from an accelerator. The invention described herein was made in the course of, or under, Contract W-7405-eng-48 with the United States Atomic Energy Commission.

Target positioners, which insert a target into the beam path of accelerated particles and then rapidly remove the target from the beam after a desired exposure time, are extensively used for internal beam bombardments in large particle accelerators such as the United States Atomic Energy Commissions Bevatron at Laurence Radiation Laboratory, Berkeley, California. US. Patent No. 2,798,- 178 issued to H. G. Heard et 211., July 2, 1957, for Accelerator Target Positioner and US. Patent No. 2,964,710 issued to K. F. Stone et 211., Dec. 13, 1960 for Accelerator Target Positioner and Control Circuit disclose prior forms of such devices. These prior target positioners have used the field of the accelerator magnet to interact with targetdrive coils which flip the target into and out of the beam. This flip-coil technique provides a very rapid motion which is essential in order to raise and lower the target in the shortest possible time. Many targets are quite fragile however and cannot withstand the shock of an abrupt stop to such rapid motion. Consequently, various damping techniques have been employed, the most successful of which also use the accelerator magnet field to provide a decelerating force toward the end of the target motion to bring the target more gently to rest.

Current physics experiments often require a target to be stationed outside the field of the accelerator magnet for bombardment with an extracted beam. Under these conditions the magnetic field of the accelerator is not available for the coil drive. In order to have the high speed provided by a flip-coil type target drive in this case an independent and isolated magnetic field such as from a stator must be used. Furthermore, whereas the use of the accelerator field provided an inherent synchronization of the target exposure 'with the beam pulse, such synchronization must now be incorporated into such stator energization circuitry.

Summary of the invention The present invention is a high speed target positioning device of the flip-coil type for use in the external beam of a charged particle accelerator. The flip-coil comprises two separate and independent coils which are sandwiched together and mounted along one edge of a rotor shaft 3,431,502 Patented Mar. 4, 1969 placed in a stator field. The separate coils are alternately energized to provide the reverse torque for insertion and withdrawal of the target from the beam. A novel linkage arrangement between the flip-coil and the target mount converts a uniformly high speed rotation of the coil drive into a gradually accelerated and decelerated high speed target motion. Very rapid target positioning is thereby achieved while shock damage to fragil targets is eliminated.

It is an object of the present invention to provide means for irradiating target materials in an external beam from a charged particle accelerator.

It is a further object of this invention to provide means for positioning a target into and out of an extracted beam from a charged particle accelerator at very high target positioning speeds.

It is another object of this invention to provide for high speed insertions of a target into an extracted charged particle beam and for high speed withdrawal of a target from such beam without whiplash, rebound, or chatter of the target.

It is a still further object of the present invention to provide means for programming the time of insertion of a target into a high energy particle beam extracted from an accelerator and also the time of the withdrawal of the target from such beam thus providing for target irradiation during the desired portion of a beam pulse.

It is still another object of this invention to provide very high speed target positioning means whereby the irradiation of several targets in timed succession within a specific beam pulse of an extracted beam from a charged particle accelerator is possible.

Brief description 0] the drawing The invention will be best understood by reference to the drawing of which:

FIGURE 1 is a perspective view of mechanical elements of the target positioner, and

FIGURE 2 is a schematic and block diagram of the electrical circuit associated with the apparatus of FIG- URE 1.

Description of the preferred embodiments Referring now to the drawing and particularly to FIGURE 1, a target 11 is mounted on a retaining bar 13 which is supported in a horizontal position by being pivoted at the upper end of two equal length radial pivot arms 14 and 16 adjacent the particle beam 12. Arm 14 is supported at its lower end by attachment to the end of a shaft 17 which is mounted horizontally and journaled in bearing blocks 18 and 19 secured to a flat base 21. The lower end of the other arm 16 is secured to the end of a second shaft 23, shorter than and parallel to shaft 17. Shaft 23 is journaled in bearing blocks 24 and 26 on the base 21 with thedistance between the shaft 17 and 23 being equal to that between the pivot mountings of bar 13 on arms 14 and 16.

A second radial pivot arm 27 is secured to the opposite end of shorter shaft 23 and is inclined toward the direction from which the beam 12 approaches, thus it is not parallel to arm 16. A similar pivot arm 28 is secured to shaft 17, parallel to and in the same plane of rotation as arm 27. A pivot bar 29 connects the tops of arms 27 and 28.

At the remaining end of longer shaft 17, an additional arm 31 is secured to have an approximate 45 inclination toward the oncoming beam 12. A connecting rod 32 conples radial arm 31 to a crank 33 which is mounted on the rotor shaft 34 of a motor 36. The respective lengths of crank 33 and arm 31 are proportioned so that rotation of the crank 33 through rotates arm 31 through only 90, as indicated by the phantom elements 33 and 31' in the figure. For this to be the case, the length of arm 31 must be greater than that of crank arm 33 by a factor of 1.414. The linkage provided by arm 31, rod 32, and crank 33 act to convert an essentially uniform high speed rotation of the rotor shaft 34 into a gradually accelerated and decelerated high speed rotation of the target moving apparatus. For this conversion to be optimum, it should be noted that connecting rod 32, crank arm 33, and the rotor shaft 34 axis are in common alignment at either of the rest positions. The 90 rotation of arm 31, transmitted through shaft 17 and target mounting arms 14 and 16, thus swings the target 11 downward out of the particle beam 12.

The drive motor 36 has a rectangular magnetic core 37, provided with a bore 38 therethrough to accommodate the rotor shaft 34 therein. The flip-coil 40 is comprised of a pair of rectangular rotor coils 39 and 41 sandwiched together and secured along one leg thereof to the rotor shaft 34. A semi-circular magnet gap 42 for the opposite leg of the flip-coil 40 penetrates through the core 37, coaxially positioned with respect to bore 38. Two identical field coils 44 are disposed in rectangular grooves 43 provided around the core 37 below the ends of gap 42. The field coils 44 establish a magnetic flux pattern within core -37 and across gap 42 as indicated by the dashed lines 45. Upon energization of the flip-coil 40, as will hereinafter be described, the resulting magnetic force turns the coil through the 180 rotation thereby providing the torque to rotor shaft 34. Two glass epoxy strips 46, placed one on top of each field coil 44 at each end of gap 42, serve as stops for the coil 40 at each end of the 180 travel.

Referring now to FIGURE 2, the field coils 44' are shown connected in parallel with one of the common terminals coupled to the positive side of a power supply 47. The other common terminal is connected through a resistor 48 to the emitter of a transistor 49. The collector of transistor 49 is connected to the negative side of power supply 47 and the base terminal thereof connects through a resistor 51 to the movable contact arm 52' of a relay 52. The relay 52 functions as a single-pole double-throw switch controlling the energization of the field coils 44'. One contact terminal of relay 52 couples directly to the negative side of power supply 47, the other contact being coupled to the positive side through a resistor 53. In the unactivated condition relay 52 is normally closed to this latter contact (as shown). A diode 54 and a resistor 56 are connected in series across the terminals of the field coils 44 to dissipate the field energy when the circuit through the coils is opened.

The separate rotor coils 39' and 41' which comprise the flip-coil 40 of FIGURE 1 are connected in series with the common junction therebetween coupled to the negative side of power supply 47. Coil 39' is connected such that the current direction, when supplied thereto, will cause a clockwise rotation of coil 40 in gap 42 and drive the target 11 to the up position. Coil 41' is connected in such a sense that when the current is directed through this coil the magnetic rotational force will be reversed and will drive the target to the down position.

The end of coil 39' remote from the common junction with coil 41' is connected to the collector of a transistor 57, the emitter of which is connected to the positive side of power supply 47 through a resistor 58. The transistor 57 base is coupled through resistor 59 to the common terminal of a relay 61. Relay 61 controls the energization of rotor coil 39, with the relay contact arm 61', when energized, being connected to the negative side of power supply 47 and being connected, when unenergized, to the positive side of power supply through a resistor 62.

The end of rotor coil 41 remote from the common junction with coil 39' is similarly connected to a control relay 67 through a transistor 63, the emitter being connected through resistor 64 to the positive side of power supply 47 and the base being connected through resistor 66 to the armature of the relay 67. One contact of relay 67 is connected to the negative side of power supply 47 and the other contact is connected through resistor 68 to the positive side thereof, the relay contact arm 67 being normally closed to this latter contact. A diode 69 in series with a resistor 71 and a diode 65 in series with a resistor 70 are connected across the terminals of each of the coils 39 and 41' respectively to dissipate the energy contained therein when the circuit is opened.

Considering now the control circuitry for the operation of the relays 52, 61 and 67, a start pulse generator 72 delivers a sharp pulse output 73 to a field gating circuit 76 and to a first time delay and gating circuit 77. Pulse 73 actuates the target up-drive. Start pulse generator 72 also delivers a delayed pulse 74 to the field gate 76 and to a second time delay and gating circuit 78 for the target down-drive. Such gating circuits essentially act a pulsestretchers. Start pulses 73 and 74 may be repeated at a desired rate for as long as is required.

The first start pulse 73, upon entering the field gate 76, triggers a control pulse 79 therefrom which is extended for 80 milliseconds. Control pulse 79 activates relay 52, thereby permitting current flow from power supply 47 through the field coils 44' to energize the stator field. The pulse 79, as shown in the figure, is rounded on the leading edge to indicate the time for the field to build The first time delay and gate 77, which also receives pulse 73, introduces a time delay of 20 milliseconds before producing an output control pulse 81 which extends for 60 milliseconds. Pulse 81 operates relay 61 and causes current to flow through resistor 58, transistor 57, and the rotor coil 39' which, in turn, causes the target 11 to move up into the beam 12. The 20 msec. delay in circuit 77 permits the stator field to build up before energizing the rotor coil 39 for the target drive. The pulses 79 and 81 then terminate simultaneously, the stator field subsides and the target remains in the up position.

In the present example, the delayed start pulse 74 from generator 72 occurs one second after first pulse 73 therefrom. This is the exposure time of the target to the beam and may vary according to the experiment. The delayed pulse 74 again actuates the field gate 76 causing a similar control pulse 79 therefrom to energize the stator field. A second time delay and gating circuit 78 also receives pulse 74, introduces a 20 msec. time delay and then produces a second control pulse 82, similar to pulse 81 from circuit 77. Control pulse 82 actuates relay 67, permitting current flow through resistor 64, transistor 63, and rotor coil 41 to drive the target 11 down out of beam 12. Pulses 79' and 82 also end simultaneously and the target 11 remains in the retracted position. After a given period of a few seconds another start pulse 73 occurs from generator 72 and the entire cycle is repeated, the period being chosen to coincide with the next beam output pulse of the accelerator.

Considering now a cycle of operation, beginning with the target 11 in the retracted position, the first start pulse 73 activates the stator field coils 44 and energizes rotor coil 39 to raise the target 11 into the beam. The energization of coil 39 interacting with the field produces a clockwise force on the flip-coil 40 causing it to swing very rapidly through the arc of the magnet gap 42, and momentarily forces it against righthand stop 46 in order to prevent any rebound motion. This rotation is transmitted to the crank arm 33. It will be noted however that at the start and toward the end of the crank arm rotation, most of the travel is in the vertical direction and very little horizontal translation will be transmitted to the coupling arm 32. Consequently there is relatively slow movement of the target structure in those periods. In the midregion of the crank arm 33 rotation, however, the motion is mainly horizontal and the target structure will be moved most rapidly at this time. Thus, while the flip-coil 40 and crank arm 33 sweep through the 180 of travel at a uniformly high speed, the target 11 motion experiences a gradual acceleration to that high speed followed by a gradual deceleration therefrom toward the end of its 90 of travel. Shock damage to fragile targets is therefore prevented by the gentle starting and stopping of the target motion. After the target 11 has been bombarded by the beam 12 particles, the delayed pulse 74 from generator 72 re-activates the field coils 44 and energizes rotor coil 41 in this case. Flip-coil 40 now rotates in the counter-clockwise direction and the target mount is swung downward, out of the beam.

Although the invention has been disclosed with respect to a single embodiment, it will be evident to those skilled in the art that other variations are possible within the scope of the invention. For instance, the flip-coil 40 operation may readily be altered, with the appropriate alteration of the accompanying circuitry, such that during the latter half of the respective drive cycles of the separate coils 39 and 41, the opposite coil could be energized to oppose the drive motion and add to the damping effect. Also, the flip-coil could readily be composed of a single coil, and the polarity of the energizing current directed therethrough be reversed between each successive cycle to effect the alternate up and down drives for the target motion. Accordingly, it is not intended to limit the invention except as defined by the following claims.

What is claimed is:

1. In apparatus for rapidly moving a target into a charged particle beam which is external to a charged particle accelerator, the combination comprising a rotatable elongated shaft disposed transverse to and displaced from the path of said charged particle beam, a first radial arm projecting from said shaft and rotatable therewith, a target retainer for holding said target to be irradiated by said beam mounted on the extremity of said first radial arm whereby rotation of said shaft swings said target into a position intercepting said beam, a drive shaft disposed parallel to and laterally displaced from said elongated shaft, a crank arm projecting radially from said drive shaft, a second radial arm projecting from said elongated shaft at a point therealong in the plane of said crank arm from said drive shaft, a connecting rod pivoted at one end to the extremity of said crank arm and pivoted at the other end to the extremity of said second radial arm, said crank arm, connecting rod, and said second radial arm having a first rest position where the axis of said drive shaft and the pivot points of said connecting rod describe a straight line, and drive means coupled to said drive shaft and selectively producing a reversible 180 rotation thereof whereby said target retainer is moved through an arc of 90 between a first rest position adjacent said beam and a second rest position remote from said beam.

2. Apparatus as described in claim 1 wherein the length of said second radial arm is greater than the length of said crank arm by a factor of 1.414.

3. Apparatus as described in claim 1 wherein said drive means comprises a motor having a magnetic core provided with a bore therethrough and with an arcuate opening therethrough of slightly more than 180 and coaxial with said bore, said drive shaft being rotatably mounted in said bore, a pair of rectangular field coils each disposed with an inner leg thereof wound through opposite ends of said arcuate opening with the outer leg thereof wound around the respective outer end of said core whereby said coils when energized provide an essentially radial magnetic flux pattern across said annular opening, and a rotor comprised of at least one rectangular coil and secured along one leg thereof to said drive shaft with the opposite leg thereof disposed through said arcuate opening whereby said rotor is rotatable through the 180 of said opening.

4. Apparatus as described in claim 3 wherein said rotor comprises a pair of separate and oppositely wound rectangular coils sandwiched together and mounted on said drive shaft whereby energizing a first of said coils drives said rotor in a first direction to dispose said target in said beam and energizing a second of said coils drives said rotor in an opposite direction.

5. Apparatus as described in claim 3, comprising the further combination of a pulse generator producing a first trigger pulse at a first output thereof for initiating movement of said target into said beams, said pulse generator producing a second trigger pulse at a second output thereof for initiating removal of said target from said beam, a first gate circuit having inputs coupled to said first and second outputs of said pulse generator and producing first and second extended output pulses at an output thereof in response to said respective first and second trigger input thereto, a power supply, a first power control means connected in series with said field coils across said power supply and having a control terminal connected to the output of said first gate circuit, said first power control means closing during receipt of pulses from said first gate circuit, a second gate circuit having an input coupled to said first output of said pulse generator and producing a slightly delayed output signal timed to terminate simultaneously with said first extended output pulse from said first gate circuit, a second power control means connected in series with a coil of said rotor across said power supply and having a control terminal connected to the output of said second gate circuit and sense of which coil when energized acts to rotate said target into said beam, said second power control means closing during receipt of pulses from said second gate circuit, a third gate circuit having an input coupled to said second output of said pulse generator and producing a slightly delayed output signal timed to terminate simultaneously with said second extended output pulse from said first gate circuit, and a third power control means connected in series with a coil of said rotor across said power supply and having a control terminal connected to the output of said third gate circuit the sense of which coil when energized acts to rotate said target out of said beam, said third power control means closing during receipt of pulses from said third gate circuit.

References Cited UNITED STATES PATENTS 2,599,188 6/ 1952 Livingston 31362 2,798,178 7/1957 Heard et al 313-62 2,964,710 12/1960 Stone et al 328235 JAMES W. LAWRENCE, Primary Examiner. V. LAFRAN CHI, Assistant Examiner.

U.S. Cl. X.R. 

